Why doesn't air freeze? I am in no way experienced in the Physics field so this question may seem a bit silly but i'd appreciate an answer :)
Why doesn't air freeze?
 A: At the normal pressure, 99.9% of air (nitrogen, oxygen and argon) will solidify in 55K (where the oxygen does). Below about 15K also hydrogen (0.000524% of normalized air) solidifies leaving only helium which will be a liquid up to 0K (yet will change into a superfluid about 2.17K).
A: Air does freeze just at temperatures and pressures we don't often experience.
There is an entire industry around producing and distilling liquid air.  You take air and compress it.  As a result the air increases in temperature; the air is allowed to cool. Then the air is expanded by venting it into a new chamber. The result is a much colder gas.
Through cycles of compression, cooling and venting we can get air to condense into a liquid.
If liquid nitrogen is then placed in a vacuum chamber, it will freeze.
A: There's a tiny trace of helium in air, about 5 parts in a million. The pressure isn't high enough for it to freeze no matter how cold it gets
A: The state (solid, liquid or gas) of a substance depends on the strength of the bonds made between the molecules of the substance and the (thermal) kinetic energy of the molecules.  
To break a bond between molecules requires energy and the stronger the bond the more energy is required to break the bond.  
For the gases that make up the air the kinetic energy that the molecules have is easily sufficient to supply enough energy to break any bonds between the molecules.  
If the temperature is lowered the kinetic energy of the molecules decreases and below a certain temperature the kinetic energy is not sufficient to break enough bonds between the molecules to stop them staying together in the liquid state, or even lower in temperature in the solid state.  
So the answer to you question is that temperature of the air is maintained at a sufficiently high temperature by the Sun such that collisions between molecules do not result in a permanent bond between the molecules.
Go further from the Sun where the surface temperature on the plants is lower and gases do become liquids and even solids.  
Of all the gases in the atmosphere carbon dioxide has a relatively high freezing (sublimation) temperature at $-78.5\, ^\circ \rm C$ and the lowest recorded temperature is $-78.5\, ^\circ \rm C$ in the Antarctic there is a possibility that carbon dioxide might freeze.
However, as this article points out, that is unlikely to happen because there are other factors involved.  
Given that the boiling points and freezing points tend to decrease as the molecular mass decreases an amazing fact is that water exists in its liquid and solid form on the Earth.
So really this an example of the opposite effect to that mentioned in your question.  
Hydrogen sulphide $\rm H_2S$ is a gas at room temperature and yet the lighter water molecule $\rm H_2O$ can exist as a liquid at such a temperature.  
This illustrates the importance of the strength of the bonds between molecules.
Water molecules have a large dipole moment (asymmetric charge distribution) which results in a relatively much stronger (hydrogen) bond between water molecules as compared to those between hydrogen sulphide molecules.
So more thermal agitation (corresponding to a higher temperature) is needed to break the bonds between water molecules than those between hydrogen sulphide molecules.
A: Air is composed of several different gases, but the most important ones are nitrogen and oxygen. Since these components have very low melting points, you won't see air freeze any time soon on Earth, but on the other hand, liquid nitrogen is produced routinely for all kinds of purposes (like for physics students to toy with during boring lab work).
The question is now reduced to why nitrogen and oxygen are present in gaseous forms at temperatures we are accustomed to and why they have such low melting points.
To answer the first question, temperature on Earth is what it is thanks to the Sun's light that warms us, the Earth's own geothermal heat, but also because of the greenhouse properties of a lot of gases in our air. (So air has a "hand" in warming itself if you want. :p)
For the second, the metling points of nitrogen and oxygen are so low because the molecules that make up these gases are so tiny and loosely bound. It takes little energy to excite them, therefore at "room temperatures" they are extremely mobile and are basically moving around in straight lines until they bump on each other or on anything else. The characteristic behaviour of molecules in liquids and solids is much less mobile.
A: Air is not a pure substance.  Only two gases at one atmosphere can freeze out in extreme cold conditions. The gases are water vapor (rain, snow, and sleet) and Carbon Dioxide.  I have not heard of CO2 sublimation ~ -79C.  Temperatures have been cold enough (~ -90C) but the altitude is higher than sea level, so I am not sure that this had ever happened in recorded history.  The other gases freeze out or rain out at much lower temperatures. These gases include the elements Oxygen, Nitrogen, Argon, Xenon, Hydrogen, Helium, and Neon. 
